The crown wheel and pinion are the final drive gears in a rear wheel drive car. The gears are called Hypoid gears.
Hypoid gears are a special design of spiral bevel gears which have the centreline of the pinion below the centerline of the crown wheel. One advantage for passenger cars is that the proppeller shaft or input shaft is lower and the rear floor can be made flatter. Hypoid gears are used in commercial vehicles as well. The tooth shape of these gears provide a greater area of tooth contact and so greater strength, although correct gear adjustment is needed to prevent gear noise.
Hypoid gears have high tooth loading. The teeth do not merely roll against each other, but have a wiping action as well. this requires a special lubricant which must have high pressure additives. Gear oil in the housing is picked up by the crown wheel and circulated to the pinion and bearings.
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Differential:
The differential divides torque between the rear wheels and also enables the driving wheels to rotate at different speeds when turning. When turning the outer wheel turns further than the inside wheel. With out a differential, the driving wheels would have to skid every time the car is steered. As well as causing tyre wear, the vehicle would be almost impossible to steer.
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| Differential gears. |
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